Examples of conventional vehicular (automobile) brake systems known are ones equipped with a servo unit such as a vacuum booster or a hydraulic booster. In recent years, electric boosters that are driven by an electric motor have become popular (see, for example, Patent Document 1).
The electric booster disclosed in Patent Document 1 includes a main piston that moves back and forth in accordance with an operation of a brake pedal, a cylindrical booster piston that is externally engaged in a manner displaceable relative to the main piston, and a rotation-linear-motion converting mechanism like a ball screw that transmits the rotational force of an electric motor to the booster piston as a booster thrust force.
According to this electric booster, the main piston and the booster piston are utilized as pistons for a master cylinder, and respective front ends of those pistons are in communication with the pressure chamber of the master cylinder. Hence, a brake fluid pressure is produced in the master cylinder by a thrust force input into the main piston from an operator through the brake pedal and the booster thrust force input into the booster piston from the electric motor.